1. Field of the Invention
The present invention relates to improved sensors and also to an improved sensor and data recording unit. The invention has been developed primarily for use in the acquisition of seismic data via the sensing of seismic signals at the seafloor and will be described hereinafter with reference to this application. However it will be appreciated that the invention is not limited to this particular field.
2. Background
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Instruments for recording seismic data at the seafloor have been in use for many years. Such instruments were developed initially for earthquake monitoring and were subsequently modified for other applications such as geophysics and petroleum reservoir development research. They are generally referred to as Ocean Bottom Seismometers and may be deployed to a water depth of up to 6000 meters.
Ocean Bottom Seismometers typically include electronics such as a data logger, an accurate clock and batteries. Such prior art units may also include recovery means such as a weight release mechanism. The sensors typically utilise geophones and/or hydrophones.
It has been appreciated by the inventors of the present patent application, that some prior art ocean bottom seismometers exhibit at least some of the following performance characteristics:                poor frequency response due to the mass of the sensor/recorder unit;        poor frequency response due to inadequate transfer of seismic vibrations from the seafloor to the sensors;        excessive weight and complexity, particularly in relation to those prior art units which utilise a relatively heavy chassis which is required to land vertically on the seafloor after which a sensor is lowered onto the seafloor by a mechanical release mechanism;        excessive deployment complexity;        instability of the sensor, particularly when deployed onto an irregular seafloor;        excessive vulnerability to damage from external impacts; and        non-uniform sensing of seismic waves due to lack of sensor symmetry, particularly in relation to prior art sensors encapsulated in a spherical housing and those sensors having an elongate cylindrical body which, once deployed, may be free to roll in a direction perpendicular to the cylindrical axis.        
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.